1. Field of the Invention
The present invention relates to a method of pattern delineation and, more particularly, to a method of delineating a pattern while less affected by beam deflection distortion.
2. Description of Related Art
In a charged-particle beam lithography system, such as an electron-beam lithography system, a charged-particle beam is directed at a given position on a material on which a pattern is to be delineated according to data about the pattern, the material being coated with a photosensitive material. Thus, the pattern to be delineated is written.
In this charged-particle beam lithography system, a chip pattern having a polygonal pattern is divided into simple rectangular or trapezoidal pattern segments. The pattern is written on a material according to data about the pattern segments. A triangular pattern segment is recognized as a trapezoidal pattern segment having an upper or lower base having a length equal to 0.
Where a polygonal pattern is divided into rectangular or trapezoidal pattern segments, the smoothness of the original polygonal figure is lost as shown in FIG. 5. In FIG. 5, the solid line indicates the original, undivided polygonal pattern. The broken line indicates pattern segments obtained by a division. The symbol x indicates grid points representing units of measure used where data is created. For ease of understanding, the pattern segments obtained after the division are drawn to be slightly shifted.
As shown in FIG. 5, a division line L1 intersects the hypotenuse L2 of the polygonal pattern. The position of the intersection of L1 and L2 is not always at any one of the grid points of the coordinate system by which the geometrical figure is represented. Rather, the intersection is approximated by the closest grid point, which, in turn, becomes a vertex of a rectangular or trapezoidal pattern segment obtained by the division. That is, it is meant that the original polygonal pattern is not always regained if the rectangular or trapezoidal pattern segments are combined.
Furthermore, in a charged-particle beam lithography system, such as an electron-beam lithography system, movement of the written material is normally utilized in addition to beam deflection to delineate a pattern of a size exceeding the deflection region (e.g., a chip pattern) of the beam deflector.
Where such a pattern delineation is carried out, it is necessary to divide a chip pattern into pattern segments according to each field, where delineation can be performed only by beam deflection without moving the material. This may be hereinafter referred to as field division.
This method is illustrated in FIG. 6. When a polygonal pattern as shown at step (a) is divided according to individual fields, a geometry including the fields F11-F22 as shown at step (b) is obtained. The polygonal pattern shown at step (a) is divided by a field boundary line L3 extending in the X-direction and a field boundary line L4 extending in the Y-direction. The symbol ▴ shown in the fields indicates the origins of the fields, i.e., positions of the fields.
We have proposed the following lithography method in order to prevent the smoothness of the original geometrical pattern from being lost due to the division of the chip pattern and to smoothly stitch together pattern segments spanning adjacent fields.
A chip pattern is divided using two kinds of fields which are slightly shifted with respect to each other in field boundary line position. A chip pattern segment obtained in one field is divided into rectangles or trapezoids by line segments extending parallel to the X-axis (hereinafter referred to as the X division). A pattern segment obtained in the other field is divided into rectangles or trapezoids by line segments extending parallel to the Y-axis (hereinafter referred to as the Y division). The pattern segments are overlappingly written at the same position on the same material with an electron beam dose that is a half of the dose normally used. This is hereinafter referred to as the XY overlap lithography.
The prior art XY overlap lithography is illustrated in FIG. 7. A case in which a polygonal pattern as shown at step (a) is delineated is now assumed. The pattern is divided into pattern segments by a first method of field division as shown at step (b). The pattern segments are divided by X division to obtain a geometry as shown at step (c).
On the other hand, the same polygonal pattern shown at step (a) is divided using a field having a field boundary line slightly shifted with respect to the field boundary line of the field shown at step (b) as shown at step (d). This method of division is referred to as the second method of division. The resulting pattern segments are divided by Y division to obtain a geometry as shown at step (e).
The geometries shown at steps (c) and (e) are laid to overlap each other and delineated lithographically. As a result, a lithographic pattern as shown at step (f) is obtained. Also, in this case, for ease of understanding, the finally obtained pattern is drawn to be shifted slightly.
Where a pattern is written on a material by deflecting an electron beam, beam deflection distortion is induced. The magnitude of the beam deflection distortion varies depending on the position within the field. Therefore, the positional accuracy and dimensional accuracy of the delineated pattern vary depending on the position within the field at which the pattern is delineated. Generally, the beam deflection distortion is smaller at the center of the field and larger at marginal portions.
FIG. 8 is a perspective view of a part of an electron-beam lithography system. A chip pattern 2 is delineated on a material 1 (such as a photosensitive material). An electron beam deflector 3 deflects the electron beam EB to write the pattern on the material 1. A field 4 indicates a range in which lithography can be performed only by deflection of the beam EB. The material 1 is moved in the direction indicated by the arrow.
If pattern segments spanning adjacent fields are written by such an electron-beam lithography system within each field, the smoothness of stitching between the pattern segments spanning the adjacent fields is deteriorated due to the positional accuracies and dimensional accuracies in the fields.
FIG. 9 shows the manner in which patterns are written across fields. Lithographic fields F1 and F2 are present on a material. The black arrows indicate beam deflection distortion. In the case of a pattern delineated at or near the center of a field as indicated by B, no step is produced. In contrast, steps are produced on a pattern delineated across fields as indicated by C.
It is difficult to completely remove the beam deflection distortion in such a field. It is necessary to suppress variations in positional accuracy and dimensional accuracy of pattern due to such beam deflection distortion by some additional method.
However, in the above-described method of XY overlap lithography, the positions in the fields where pattern segments obtained respectively by X division and Y division are delineated are different only slightly. Consequently, the process is greatly affected by the beam deflection distortion.